In the art of respiration devices, there are well known a variety of respiratory masks which cover the nose and/or mouth of a human user in order to provide a continuous seal around the nasal and/or oral areas of the face such that gas may be provided at positive pressure within the mask for consumption by the user. The uses for such masks range from high altitude breathing (i.e., aviation applications) to mining and fire fighting applications, to various medical diagnostic and therapeutic applications.
The prior art of such masks includes U.S. Pat. Nos. 2,939,458; 2,855,924; 2,166,164; 2,706,983; 4,167,185; and 4,414,973. Additional prior art includes British patent specification 8,372,50 and Italian patent 321483.
One requisite of such respiratory masks has been that they provide an effective seal against the user's face to prevent leakage of the gas being supplied. Commonly, in prior mask configurations, a good mask-to-face seal has been attained in many instances only with considerable discomfort for the user. This problem is most crucial in those applications, especially medical applications, which require the user to wear such a mask continuously for hours or perhaps even days. In such situations, the user will not tolerate the mask for long durations and optimum therapeutic or diagnostic objectives thus will not be achieved, or will be achieved with great difficulty and considerable user discomfort.
The prior are includes at least two types of respiratory face masks for the types of applications mentioned above. The most common type of mask incorporates a smooth sealing surface extending around the periphery of the mask and exhibiting a generally uniform (i.e., predetermined or fixed) seal surface contour which is intended to be effective to seal against the user's face when force is applied to the mask with the smooth sealing surface in confronting engagement with the user's face. The sealing surface may consist of an air or fluid filled cushion, or it may simply be a molded or formed surface of a resilient seal element made of an elastomer such as plastic or rubber. Such masks have performed well when the fit is good between the contours of the seal surface and the corresponding contours of the user's face. However, if the seal fit is not good, there will be gaps in the seal-to-face interface and excessive force will be required to form the seal member and thereby attain a satisfactory seal in those areas where the gaps occur. Such excessive force is unacceptable as it produces high pressure points elsewhere on the face of the user where the mask seal contour is forceably deformed against the face to conform to the user's facial contours. This will produce considerable user discomfort anywhere the applied force exceeds the local perfusion pressure, which is the pressure that is sufficient to cut off surface blood flow. Ideally, contact forces should be limited between the mask and the user's face to avoid exceeding the local perfusion pressure even at points where the mask seal must deform considerably.
The problem of seal contact force exceeding desirable limits is even more pronounced when the positive pressure of the gas being supplied is relatively high or is cyclical to high levels. Since the mask seals by virtue of confronting contact between the mask seal and the user's face, the mask must be held gainst the face with a force sufficient to seal against leakage of the peak pressure of the supplied gas. Thus, for conventional masks, when the supply pressure is high, headstraps or other mask restraints must be tightly fastened. This produces high localized pressure on the face, not only in the zone of the mask seal but at various locations along the extent of the retention straps as well. This too will result in severe discomfort for the user after only a brief time. Even in the absence of excessive localized pressure points, the tight mask and headstraps often may become extremely uncomfortable and user discomfort may well cause discontinued cooperation with the regimen.
A second type of mask, which has been used with a measure of success, particularly in aircraft and rescue applications, incorporates a flap seal of thin material so positioned about the periphery of the mask as to provide a self-sealing action against the face of the user when positive pressure is applied within the mask. With this type of sealing action, the forces which serve to hold the mask in confronting engagement on the face of the user are much lower than with the first type of mask described above. If the flap seal is capable of conforming to the contours of the user's face without forming leak paths, the mask can be used with retention straps which exert little or no net force to push the mask against the user's face. Thus, the overall sensation of constraint and confinement is dramatically reduced for the user. Such a mask, when properly adjusted, can be adapted to any positive internal mask pressure. The sealing flap will be self-sealing as long as there is no looseness in the strapping arrangement which would allow the mask to move away from the face further than the reach of the sealing flap when subjected to internal pressure.
Among the potential limitations of the second described masked type are two of note. First, the sealing flap seals by laying flat against the user's face throughout its length. This action requires a close match between the contours of the face and those of the seal. If the match is not good, the seal will be ineffective. Secondly, the normal response of one applying the mask to a user's face is to push the mask harder against the user's face if the mask does not seal. With the typical flap seal-type mask, increasing contact pressure against the user's face will not help to form an effective seal if the flap seal does not initially fit well to the facial contours. It may, however, lead to patient discomfort and other problems as described above.
Further regarding the above-described second mask type, some of the principal problems one encounters when trying to apply the self-sealing flap concept to the design of the respiratory mask are related to the location of relative low points and high points in the facial contours of the user relative to the shape or contour of the flap seal surface. If the seal surface does not contact the user's face at the relative lower points, then excessive gas leakage will occur thus preventing sufficient internal gas pressure to develop to activate the sealing action of the seal flap at the low points. In the prior art, this problem has been solved for some applications by providing a variety of masks with differing seal flap shapes, sizes and contours. For example, for aircraft breathing masks, especially where expense is not a critical factor, wide variety of mask shapes and sizes may be provided to give the individual users an opportunity to find a mask offering good fit. In other breathing mask applications such as clinical use, where economic considerations may dictate a mask having the capability to accommodate a wide variety of facial sizes and contours, prior flap type seal structures have not generally been able to provide the requisite versatility.
A related problem with flap seal mask structures concerns the high points of the user's face, where the seal flap may tend to distort or collapse and fold in on itself, thus creating a channel for gas leakage, when pressure is applied in order to effect a seal at adjacent relative low points on the user's face. Even where the section thickness of the seal flap is very limited, and the material is very soft and flexible, the internal gas pressure cannot overcome some such seal flap distortion to provide the desired self-sealing.
It is also known to provide the above-described flap seal type mask with integrally molded structural elements to allow for adaptation of the mask seal to a wide variety of facial shapes and contours. More specifically, it is known to provide properly positioned, resiliently deformable upstanding ribs or similar structural means, preferably located internal of the flap seal and integral therewith, to maintain the flap seal in the proper positional relationship with respect to the face of the user even when the seal and associated ribs are resiliently deformed, so that the positive internal mask pressure will provide the requisite self sealing characteristic. This allows a single mask structure to be adapted to a wide variety of facial contours whereby only a few different masks are required to service a wide range of facial contours normally encountered. In theory the self-sealing flap arrangement allows retention strapping to be used without application of high retention forces against the face or about the head of the user. However, many emergency medical technicians have little if any practical experience or training in the proper use of such masks. As a result, the mask straping may often be drawn too tightly and unnecessarily cause considerable patient discomfort. Soft strapping material preferably is utilized with such masks, so that a comfortable sealing action of the seal flap can be achieved and high user tolerance is realized for extended periods. However, even this expedient will not alleviate the discomfort of tightly drawn strapping.
The applicant herein has been a part to prior development of a mask generally of the above-characterized type and including a generally annular seal comprised of a peripheral sidewall having an inturned flexible flap seal adjacent a free end thereof, with the inturned seal being configured for confronting sealing engagement with a user's face as above described. Spaced about the peripheral seal wall are plural, upstanding, flexible ribs which serve to support the peripheral wall and an inturned portion of the seal member located generally outward of the face-engaging surface portion of the seal flap. The described seal structure is intended to permit the flap seal and peripheral sidewall to distort without experiencing any mode of seal defeating deformation such as crimping, buckling, folding or other modes of collapse. In this seal structure, the structural support ribs are located and configured in a manner to provide adequate seal flap support where seal deformation is not required (i.e., at the "low" points of the contours of the user's face) and to resiliently deform in a manner to permit easy and uniform distortion of the seal flap in those areas where distortion is necessary to accommodate "high" points on the contours of the user's face. This above described mask seal structure is considered to be part of the prior art.